Facebook2 1305. Integer to English Words

Code(Language:C++)

class Solution {
public:
    /**
     * @param num: a non-negative integer
     * @return: english words representation
     */
    std::vector<string> levels = {"Thousand", "Million","Billion"};
    vector<string> nums1 = {"One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine","Ten", "Eleven", "Twelve", "Thirteen", "Forteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"}; 
    vector<string> nums2 = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"}; 
    string numberToWords(int num) {
        // Write your code here
        string res = "";
        if(num == 0){
            return "Zero"; 
        }
        int level = 0; 
        while(num > 0){
            int cur = num % 1000; 
            string tmp = ""; 
            if(cur > 0){
               int lower = cur % 100; 
               if(cur / 100 > 0){
                  tmp = nums1[cur / 100 - 1] + " Hundred" + " "; 
               }
               if(lower != 0){
                   if(lower < 20){
                      tmp += nums1[lower - 1] + " "; 
                   }
                   else{
                       int digit1 = lower % 10; 
                       tmp += nums2[lower / 10 - 2] + " "; 
                       if(digit1 != 0){
                          tmp += nums1[digit1 - 1] + " "; 
                       }
                   }
               }
            }
            if(tmp != "" && level > 0){
                tmp += levels[level - 1] + " "; 
            }
            res = tmp + res; 
            num /= 1000; 
            level++; 
        }
        res.pop_back(); 
        return res; 
    }
};

class Solution {
public:
    /**
     * @param num: a non-negative integer
     * @return: english words representation
     */
       string numberToWords(int num) {
        string res = convertHundred(num % 1000);
        vector<string> v = {"Thousand", "Million", "Billion"};
        for (int i = 0; i < 3; ++i) {
            num /= 1000;
            res = num % 1000 ? convertHundred(num % 1000) + " " + v[i] + " " + res : res;
        }
        while (res.back() == ' ') res.pop_back();
        return res.empty() ? "Zero" : res;
    }
    string convertHundred(int num) {
        vector<string> v1 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        vector<string> v2 = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        string res;
        int a = num / 100, b = num % 100, c = num % 10;
        res = b < 20 ? v1[b] : v2[b / 10] + (c ? " " + v1[c] : "");
        if (a > 0) res = v1[a] + " Hundred" + (b ? " " + res : "");
        return res;
    }
};